In the oil and gas industry, it is necessary when abandoning a well or isolating different zones of the well to seal an annular space between a central string of pipes and a surrounding protective string of pipes. To seal the annular space, it is desired to punch a hole in an innermost pipe of a series of at least two concentric strings of pipe so as to only penetrate the innermost pipe string without damaging or penetrating any other surrounding pipe strings in the well. Further, the punched pipe must not be fractured or damaged except for a limited size hole punched in a side wall.
Since it is impossible to effect a perfectly vertical well bore, there is always some degree of offset from a perfectly vertical orientation of the well bore to produce a high and a low side. Therefore, previous to the present invention, a zero degree phase gun has been lowered into a well bore and, due to the effect of gravity, the gun lays along one side (the low side) of a central casing of the well bore. A magnetizer holds the gun against the one side of the steel central casing.
The zero degree phase gun explodes a charge against the low side of the well bore.
Cement is then fed through the central casing and passed through the opening produced by the explosive charge to fill an annular gap between a central casing and a surrounding protection casing. It is important to seal the annular gap between the central casing and only the next adjacent protective pipe string so as to seal any naturally produced gases or to isolate different zones of the well bore. If an additional string of pipe is perforated by accident, it is not possible to assure that gases are being sealed by the filling with cement of the annular gap between the innermost and the most adjacent string of pipe.
Further, with a zero degree phase gun, an electrical line must be fed down through the well casing with a magnetic decentralizer, which ensures contact of the gun with a side wall of the steel well casing. As mentioned above, this contact with the well casing will be, due to gravity, on the low side of the well casing. Cement therefore poured through the well casing which is supposed to pass through the opening produced by the explosive charge into the space between the well casing and the protective casing does not usually fill this space on the high side of the well casing, leaving pockets or "channelling" through which it is possible for natural pressure to escape to the surface.
Examples of perforating charges lowered into a pipe string are disclosed in U.S. Pat. Nos. 4,688,640 to Pritchard, Jr., 4,552,234 to Revett, 3,426,850 to McDuffie, Jr., 3,280,913 to Smith, 4,352,397 to Christopher, 4,760,883 to Dunn, 3,011,550 to Kenneday, and 3,366,188 to Hicks.
The most common method of punching holes today is to use a 1 11/16 inch outside diameter steel carrier gun with a zero degree phase and a 1 11/16 inch magnetic decentralizer which is magnetized on one side. The magnet and perforating charge must face the same direction. This tool automatically finds the low side of the well bore and always perforates the casing on this low side. This results in a poor cementing of the annular space between a central string of pipe and a surrounding protection casing.